1. Field of the Invention
This invention relates to a semiconductor laser element useful for wavelength multiplex light information transmission, processing, recording, etc., and in particular to a semiconductor laser element emitting laser lights of different wavelengths by controlling the magnitude of an electric current poured thereinto.
2. Related Background Art
In recent years, the advance of technology in the fields of optical communications and optical information processing is striking. For example, in the field of communications, wavelength multiplex transmission and frequency modulation transmission systems are being studied for the expansion of transmission band. Also, in the field of optical recording, there are being carried out studies on the improvement in the recording density by making wavelength multiplex. Therefore, a wavelength varying function of high performance has come to be required of semiconductor laser elements used in such fields.
As a variable wavelength semiconductor laser element, one using the fundus level and the high-order level in a quantum well layer is proposed in U.S. Pat. No. 4,817,110. Also, a variable wavelength semiconductor laser element having a plurality of quantum well layers of different compositions and thicknesses provided in one and the same active layer is proposed in Japanese Laid-Open Patent Application No. 63-312688.
The principle of wavelength variation in these prior-art semiconductor lasers will hereinafter be described.
First, in the case of an active layer comprising a quantum well structure, it is well known that as the electric current applied thereto is increased, the peak of the gain in the active layer shifts from the low energy gap (long wavelength) side to the high energy gap (short wavelength) side.
In a semiconductor laser element having an active layer of such quantum well structure, when an electric current is uniformly applied to the direction of resonance, a long-wavelength light is oscillated. That is, the element emits a light corresponding to the low energy gap with the total loss in the resonator being reduced.
Also, if an electric current is non-uniformly applied to this semiconductor laser element, a short-wavelength light is oscillated. At this time, the gain becomes small in the loss area and therefore, a great deal of electric current is poured into make the gain correspondingly great in the gain area. Particularly, the gain on the short wavelength side of the loss area is very small and therefore, the electric current applied to the gain area is made so great that the total gain in the resonator becomes greatest in the short-wavelength light.
Here, the uniform application in the direction of resonance refers to a state in which an electric current is applied in at an equal current density in the direction of resonance of the light in the active layer and the light in an active waveguide in which electrons concentrate. In contrast, the non-uniform application refers to a state in which an electric current is applied into respective areas in the entire resonator at different current densities even if in a certain area, the current density is uniform per the length in the direction of resonance. Also, to make the electric currents applied to the respective areas different, a separating portion is necessary between the areas. No electric current is applied to this separating portion even during the uniform application. However, if this separating portion is of a length of 10 .mu.m or less, when the expanse of the carrier in the active layer is considered, the electric current can be regarded as being substantially uniformly applied. Thus, the prior-art wave-length varying system has been such that a long-wavelength light is oscillated during the uniform application and a short-wavelength light is oscillated during the non-uniform application.
However, the above-described example of the prior art has suffered from a problem that the difference between the current value during the uniform application and the current value during the non-uniform application is very great. For example, the oscillation threshold value current during the non-uniform application is usually several tens of times as great as that during the uniform application. Therefore, the temperature of the element rises to thereby shorten the life of the element.
Also, in the above-described example of the prior art, the length of an absorption area is very important, and there has happened a case where if the absorption area during the non-uniform application is long, the amount of loss of the short-wavelength light becomes great and the short-wavelength light will not be oscillated however greatly the gain of the active area may be increased. This is because the gain on the short wavelength side of the loss area is remarkably reduced with a decrease in the application of electric current, while the gain on the short wavelength side of the gain area exhibits a saturation tendency with an increase in the application of electric current and therefore there may arise a state in which the loss of the short-wavelength light in the loss area cannot be completely made up for.